Electron tube structure



Oct. 11, 1955 SLOAN ELECTRON TUBE STRUCTURE 2 Sheets-Sheet 1 Filed NOV.5 1951 H IM-HHI HH R mm H H -HIM-H INVENTOR. DAV/D H. SLOAN A TWP/V516Oct. 11, 1955 D. H. SLOAN 2,720,611

ELECTRON TUBE STRUCTURE Filed NOV. 5, 1951 2 Sheets-Sheet 2 [l5 I27 27'I23 I13 I l I I FIIE E IN VEN TOR. 0.4 we H. SL omv BY. W W

United States Patent Ofifice 2,720,611 Patented Oct. 11, 1955 ELECTRONTUBE STRUCTURE David H. Sloan, Berkeley, Calif., assignor to ResearchCorporation, New York, N. Y., a corporation of New York ApplicationNovember 5, 1951, Serial No. 254,913

16 Claims. (Cl. 3156) This invention relates to electronic tubes and,particularly, to tubes adapted for the generation or amplification ofelectrical waves in the microwave or centimeter region, specificallytubes of the resnatron type such as are disclosed in the prior PatentNo. 2,424,002, granted July 15, 1947, and by various other patents andprior applications of the present inventor, among these beingapplication Serial No. 771,852, filed September 3, 1947, now Patent No.2,641,734, and application Serial No. 221,017, filed April 14, 1951, nowPatent No. 2,653,273.

Tubes of this general type are characterized by the fact that theyemploy density-modulated electron streams (as distinguished fromvelocity-modulated streams such as are developed by klystron tubes)developed in cavity resonators tuned substantially to the desiredfrequencies of operation. Control in such tubes is exercised by a gridstructure formed in the wall of the cavity, with a cathode in closeproximity to the grid so that the changing shape of the grid-cathodefield in the course of the oscillation controls the electron flow. Ingeneral, and specifically in the case of the present invention, theanode which receives the electron stream is positioned in a separatecavity tuned to the same frequency as the cathode-grid cavity. In tubesof this character the transit time of the electrons, at least in theanode cavity, is of the order of magnitude of the period of oscillationof the device; the electrons acquire energy by acceleration in aunidirectional field and deliver this energy to the anode cavity bydeceleration in the oscillating field set up within the cavity.

Resnatrons have been built using various numbers of electrodes, i. e.,as triodes, tetrodes, and pentodes. They are primarily high-powerdevices, delivering many kilowatts or even megawatts at the extremelyhigh frequencies at which they operate. It has been found that the mostadvantageous type of structure for many purposes is the tetrode, whereinmost of the energy is imparted to the electron stream by an acceleratingelectrode, preferably biased to the same potential as the anode.

In prior tubes using this construction, two entirely separatecavity-resonator structures have been utilized in order to permit theapplication of the proper biases to the various electrodes. One of thesecavities is the cathodegrid cavity, wherein the cathode and grid operateat the same or relatively slightly dilferent bias potentials. The secondcavity has been the accelerator-anode cavity, completely insulated fromthe first-mentioned cavity, operated at a bias potential severalthousand volts higher. The arrangement just described requires thatthere be a space between the two resonant cavities through which thedensity modulated electron stream passes. This space possesses thecharacteristics of and can act as a cavity resonator, and there istherefore a tendency to set up oscillations within it at the frequencyto which the two designedly resonant cavities are tuned. To the extentthat such oscillations are set up in the intercavity space power iswasted. By detuning the space, and by introducing damping therein, thewaste of power can be made relatively small, percentagewise. What poweris generated within resonant cavity.

this space must, however, be got rid of in some manner; a part of itwill be absorbed as heat in the device, but the most practical method ofgetting rid of it is to let it radiate into the surrounding space. Intubes of a few kilowatts rating the problems involved in permitting suchan escape are usually not serious, but where the device is generatingmany hundreds, or even several thousands of kilowatts, the radiation ofa very small percentage of the total power may cause interference, localheating of surrounding materials, and otherwise cause troubles which arevery serious indeed.

Other types of cavity-resonator tubes than the tetrode thus describedmight, to advantage, use auxiliary electrodes, in or adjacent theelectron stream, which for best operation should be operated at highfrequency potentials differing from those of the walls of an anode orcathode In the past such tubes have not been constructed because of thestructural complications involved in the provision of an additionalcavity, whether resonant or non-resonant.

One object of the present invention is to provide a tube capable ofhandling the large powers mentioned and, at the same time, to avoid thedilficulties which have hitherto been inherent in resnatron tubesemploying accelerator electrodes. Among other objects of the inventionare to provide a tube structure of the resnatron type which employs anaccelerating electrode, but wherein only two resonant cavities areemployed, with no interspace between them; to provide a tube structureof the character described wherein all of the oscillatory power absorbedfrom the electron stream appears directly in the anode cavity; toprovide a resnatron type electronic tube structure wherein the powerwasted through radiation from the tube structure itself is a minimum; toprovide an electron tube structure of the character described which maybe operated either as an amplifier or as an oscillator and wherein, whenemployed in the later manner, the anode circuit eflieiency may be maderelatively high so that power is delivered into the anode cavity inpulses occurring during the voltage peaks, thus developing the maximumpower for a given electron flow; to provide a tube structure whereby anauxiliary electrode may be located within a resonant cavity and thereoperate at any desired fraction of the oscillating potential of thecavity walls, including substantially zero, and in phase with thepotential of either wall as desired; and, in general, to provide anelectron tube structure wherein the mechanical and, particularly, theinsulation problems are reduced to a minimum.

For the purposes of this specification the invention will be consideredas it applies to a tetrode employing an accelerating electrode, thebroad principles applying to the use of any auxiliary electrode beingreadily apparent therefrom.

Considered broadly, the portion of the tube structure with which thisinvention primarily is concerned comprises a resonant cavity with theelectrode structures located at potential loops in opposite walls of thecavity, or, what is the same thing, at opposite poles of maximumelectric field concentration across the cavity when the latter isresonating in a mode corresponding to the desired frequency of operationof the device. Means are provided for directing a stream of electronsacross the cavity of the resonator between the electrodes and generallyparallel to the electric field. Within the cavity, along the path of theelectrons therethrough, is located an auxiliary electrode, e. g., anaccelerator electrode. The latter may be of a grid or mesh structure,but in tubes for the generation of very high power, to which theinvention is primarily directed, it is more practical to form anaccelerating electrode as an annulus surrounding the electron stream. Inthe form of the device wherein the stream is itself annular in form, theaccelerating electrode can include an additional annulus substantiallycoaxial with the outer annulus and just within the path of theelectrons. The auxiliary electrode is so mounted within the cavity thatits oscillating potential is substantially unaffected by conductionthrough its supports, but floats between that of the opposing walls ofthe :cavity in inverseproportion to the capacities of theauxiliaryelectrode to the walls between which it is located being midway thepotential of the walls if these capacities are equal. This resultisachieved by employing the following expedients;

First, the auxiliary electrode is supported by one or more struts .ofinherently high inductance, so that large potential differences can bedeveloped across them by relatively small oscillating currents; thismeans that .the cross sectional area of the struts should be as. smallas .is consistent with adequate mechanical :strength. Second, the,lengthof the strutsshould be such that, viewed from their .points'ofattachment, their impedance approaches infinity, i. e. they areanti-resonant. Third, Where possible, the struts should be attached tothe cavity wall at a pointsuch-that the energy fed into them from thecavity will reach the accelerating electrode in phase with the energytransmitted thereto through its capacities and there develop the samepotential. A separate bias supply for the auxiliary electrode may, ifdesired, be supplied. Ordinarily, however, whereit is an acceleratingelectrode, it is, more practical to connect its support directly to theanode structure itself .and operate it at the anode bias potential. Theremaining structure of the tube, to which this invention is notspecifically directed, may take various forms. In the case of thetetrode here chosen .for illustration of the principles, the completetube uses but two .cavity resonators having a common wall whereinthegrid structure. is located and which are tuned to oscillate. at.substantially the same frequency. .T he cavities areso proportioned thatthe cathode and grid are positioned at opposite locations of maximumpotential field concentration. in one. cavity, while grid and anode aresimilarly located in the .second,'all three electrodes being alined.

The accelerating electrode is located in the grid-anode cavity betweenthe two electrode structures. therein. With this arrangement theelectron stream enters the grid-anode. cavity directly through the grid,without any intervening. space wherein energy may be wasted. The gridand anode, of course, are operated at widely different D. .C.potentials- The. general methods of construction whereby the: necessaryinsulation may be. maintained and the. cavity substantially closed,electrically, although the :anode and. .grid' structures aremechanically spaced, are in ac cordance with the disclosures of theprior patent :and ap-- plicationsalready mentioned. a

The .abovemay be .more clearly appreciated by refer ence to thedetaileddescription of two preferred forms of the, device which follows, taken.in conjunction withthe accompanying drawings wherein:-

Figs. 1 and 1A together comprise an axialsectional view ofanaxial-fiow.resnatronernbodying the invention, the view being broken at the.anode-support end .of Fig.

. l and this .end shown separately in Fig. 1A in {order to the anodestructure. respectively. Each of these structures is a composite, butsince the general mode of con-. struction .is much the same as that usedin the resnatrons described inthe prior applications and patents whichhave already been mentioned, they will be described generally withoutgoing. into detail as .to the method of assembly ofthe parts except.where such detail: is pertinent. to: the inventionrherein covered.

Starting with the cathode structure at the left-hand end or the diagram,a central rod conductor I is surrounded by a tubular column 3, thesebeing the conductors through which current is supplied to a heating coil5. An insulating seal 7 holds these two conductors in proper relativeposition.

A slightly dished cathode-9 is supported on the end of the column3. Asleeve 11 surrounds the column 3 and chokes 41, 41' and 41 carries highfrequency chokes 13, 13. A flexible diaphragm 15 connects the end of thecolumn '3 with the outer periphery of the choke 15', and the sleeveengages the column frictionally so that, during construction, it can heslid on the column properly to tune the cavity of which the diaphragmforms one wall.

The cathode structure-is supportedfrom the more massive grid structurethrough a flange 17 secured to and surrounding the column 3 adjacent itsouter end, this flange connecting to thegrid structure through a sealring 19. The seal 19 attaches to a skirt 21 projecting from a;

massive ring 23. This ring surrounds and is attached to a tubularextension 25 projecting, toward the left of the diagram, from-the bottomof a grid cup 27, The grid cavity 29 is formed between the. bottom .ofthis cup and order to match the impedance of the guide with the im-.

pedance of the cavity. 1

The grid itself comprises a plurality of grid bars .33

positioned across. an axial aperture formed in the bottom of the gridcup 2.7. These bars are .curved so as to follow generally the contour ofthe dished face of the cathode 9, which they face directly. Due to thisformation, electrons liberated from the cathode are focused or convergedas they pass through the grid and theelectron stream entering the cavitywithin the cup is generally a converging cone in form. g

'The grid cup is secured through a pair of heavy flanges 35 and .37 to.a tubular skirt 39 within which areformed, conjointly with the. cup, aplurality of high frequency Surrounding the skirt 39 is a second skirt43 connecting through a huge 45 with a glass body 47, the right-hand endof which is re-entrant to form a sealed connection 49 with a flange 51.This flange is secured to a column 53 which supports the anode structure54. The latter is tubular, passing completely through thecolumn 53 andthe skirt 39 intothe interior ofthe gridcup. Its inner end is borne byspring fingers55 on the end .of the column 53. The .outer .end connects.through a flange 57 and metal bellows .59 with the-columns?) to form anair-tight seal. The end of. the anode tube S'Aaisclosedhy .a cap-61,which, in. the present.

case, is channelled to form a connection through which cooling liquidmay be admitted and circulated through the anode structure .via liquidconnections 63'and 65.

The anode may be advanced and retracted by means of v a threaded stud64, secured to the end of the anode. tube, by means of aninternally-threaded worm wheel 67 engaging with a tuning worm 69, theselatter port-ions being supported by a cylindrical fitting 71 mounted .tothe flange -'57. Useful. power is withdrawn from the cavity through'a'tapered wave guide 8'1.v

'The inner end of the anode tube carries the. anode proper, which is anannulus- 72 on the end of the anode column '54 and provided with arelatively sharp lip 73 facing the grid within the cavity of the gridcup.

The acceleratingfelectrode with the positioning and the anode atriaposition displaced outwardly of. theanode tuhefromlip 73. Thestrnt 7 7-secured 10km outer edge of flange 76 so as to lie almost entirelyoutside of the field between the ring 75 and the anode lip 73, but closeto the potential loop.

Despite the difference in size the anode cavity is tuned to the samefrequency as the cathode cavity, its capacity being smaller although itsinductance is greater.

In the description of the prior inventions of this same inventor it hasbeen disclosed how such cavities are eifectively closed and the escapeof energy therefrom prevented. This latter is eifected by thecombination of approximately quarter-wavelength chokes offering highimpedance in the direction away from the cavity and low impedancetransversely of the coaxial structure. In particular, in the SloanPatent No. 2,424,002 already referred to, it has been shown that thetransmission line sections going to make up this arrangement need not beof precisely one-quarter wavelength in order to achieve the effectsdesired and effectively suppress radiation from within the structure.

As a result of the construction of the tube of Fig. 1, when oscillatingin its most readily excited or principal mode, the magnetic field iscircular and coaxial with the device. The electric field isapproximately parallel to the axis in the region between the lip 73 ofthe anode and the grid, becoming radial between the wall of the grid cupand the anode column. Maxima of electric field occur at the lip 41 ofthe grid cup and between the grid and the lip 73 of the anode. Suchmaxima may be termed, for convenience, loops of potential, and thepositions of maximum concentration of field on the cavity walls thepoles of such potential loops. The tube is designed to operate inaccordance with this mode of oscillation. When so oscillating theprimary effect of the accelerator electrode, so far as the oscillationwithin the cavity is concerned, is merely to lower the frequencyslightly by increasing the capacity between grid and anode, owing to thethickness of the electrode in this dimension.

This would not be the case were any material energy to be fed to theaccelerating electrode through its supporting strut or struts. As wasindicated in the broad description of the invention such supply ofenergy is prevented by making the impedance of the strut high, as viewedfrom its point of attachment to the cavity wall. it the strut wereattached to a point on the wall that varied in oscillating potential inphase with the accelerating electrode when excited only by itscapacities, and to the same degree, when referred to a common datum, theimpedance of the strut would be immaterial since no potential difierencewould exist across it and hence no currents would be carried by it,irrespective of the impedance of the strut considered by itself. Such anideal condition is ditficult to attain in a practical type and hence theimpedance of the strut itself is made as high as possible. its diameteris made as small as is consistent with its necessary mechanicalstrength, thus raising its inductance and lowering it capacitance perunit length, so that high oscillating potentials may be developed acrossit by relatively small currents. This gives a mismatch with theimpedance of the cavity wall and with the electrode 76 as well.

Considered as portions of a transmission line system, reflections occurat both ends of the strut 77 and very little power can be transferredthrough it provided it be properly terminated. For this reason, thestrut may be considered as oscillating independently of either of thetwo structures to which it is connected; i. e., as a dipole.

Such oscillation in the strut implies that it be onehalf wavelengthlong, and that it connect to the cavity at or very near a potentialloop. In the tube of Fig. l the strut is roughly in the form of apartial helix about the tube axis, so that its length closelyapproximates onehalf wavelength at the mid-frequency of the tuning rangeof the tube, and it connects as near to the loop of po-.

rrv ID tential at the anode lip as is feasible without materiallydistorting the field between anode and accelerator.

The object in the design of this tube is to have the potential of theaccelerating electrode determined almost entirely by its relativecapacities to the grid and anode, the potentials between the acceleratorand the other two electrodes being inversely as these capacities. Aswill be shown hereinafter, in connection with the discussion of tubeoperation, it is usually desirable that the potential of the acceleratorbe allowed to swing about its bias potential in phase with the grid, sothat one-third, more or less, of 'the potential across the cavity existsbetween grid and accelerator and the remainder between accelerator andanode. The effective capacity of the accelerator to the grid istherefore made from two to three times as great as that to the anode,the energies transferred through these capacities is in inverseproportion thereto, and the potential of the accelerator will thereforefollow the grid. Hence, since the strut is one-half wavelength long,such energy as is fed through it arrives in phase with that from thegrid capacity and is simply reflected, having no material effect.

If the accelerator is in the neutral plane or swings with the anode, theresult is the same as long as the fields from the anode to theaccelerator and grid respectively are in phase. This will be the case aslong as the impedance looking into the strut from the cavity is high incomparison to that offered by the capacities of the accelerator to thewalls of the cavity. When this condition is met, the effect of theaccelerator on the cavity oscillation is almost solely that due to itsthickness and the efiect of the struts can be ignored.

If the strut be somewhat less than one-half wavelength, the result islittle changed; in effect it borrows enough capacity from the structuresto which it is connected to tune it to the half-wave value, reducing theefiective capacity between anode and accelerator to the extent of thatborrowed and increasing the oscillatory potential difference betweenthese electrodes correspondingly. Alternatively, the strut may beconsidered as a small inductive susceptance in parallel with thecapacitive susceptance of the accelerator to the anode, and hencesubtracted from the latter. A strut slightly over one-half wavelengthlong would have an opposite effect, increasing the accelerator-anodecapacity.

High input impedance to the strut can also be obtained by a quarter-wavestrut connected at a potential node. In this case it is current fed andcan be considered as one-half of a center-fed dipole. The efiect ofminor mis-tuning is therefore precisely the same as in the halfwave casealready discussed.

While struts an integral number of quarter-wavelengths long lendthemselves most readily to analysis, it should not be assumed that theyare the only means of obtaining supports whose theoretical impedanceapproaches infinity and the actual impedance of which is high. Dipolesmay be loaded with either inductance or capacity, and any point ofattachment to the cavity wall will have an apparent impedance which willapproach zero or infinity at current and potential nodes, and atintermediate points will appear inductive or capacitive depending on thepoint from which it is viewed. The strut, considered alone, will appearinductive if less than one quarter-Wavelength long, capacitive if over aquarterwavelength and less than one half. It is therefore theoreticallypossible to find some point of attachment for the strut within thecavity where, viewed from the auxiliary electrode, the impedance of thecavity will so load the strut as to make it anti-resonant, and wherethis occurs the energy transmitted through the strut Will be a minimum.

For obvious mechanical reasons the struts should be as short aspossible, which limits the choice of points of attachment to the cavitywalls. Half-wave or quarterwave struts attached at current or potentialnodes are pedance will approach infinity at some specific frequency andwhere if the-cross section of strut be-small and:

its inductanee high, small-departures frorn'this frequency 'will result-in only slight changes intheoscillafin'g' potential of the" auxiliaryelectrode, due to capacity laorrowed-fromor lent tothe strut. a V

"The general principles: underlying the design of the struts arethuswe'll known, but their application to the pro-design eraspecifictube'may 'be difficult. The cavities employed are of "suchcomplexformthat com putation of the field conformations is not feasible, andthe locat'ion of nodes.can be-predetermined only roughly. Moreover, anyindividual strut'wfl usually be assymetri' cally placed with respect tothe-cavity as a Whole, capacity cannot be expressed byany reasonablysimple function, and hence its electrical length depart from itsphysical lengthby an indeterminate amount.

Cold test is therefore preferably relied 'upon in-the final design oftubes embodying this invention. The accelerating electrode is mountedin-the-cavity of a mock-up of the-tube resonator :oninsulating supports,such as silk threads or even toothpicks, and the cavity is excited ata-desiredfrequency from an external source. position is adjusted untilthe potential swingv ofthe accelerator bears the desired ratio to thatof the anodegrid structure. Conductive struts are then inserted,connecting the parts as nearly at the desired points, as de termined byjudgment or'roug'h computation, as-poss'ibio, and of approximatelythe'proper length. The 'efiect upon the resonant frequency of the cavityis then measured. This effect should be substantially If it is not, thelength or point of attachment of the-strut is varied until its effectupon'the 'reasonant frequency of the cavity 'isreduced to negligibleproportions.

Ideally the strut shouldbe positioned perpendicular to the electricfield, i. e.,' in aunipotentia-l surface. Except in cavities that arealmost perfectly symmetrical-e. g., a right cylinder-this isimpractical. The practical expedient is to make it anti-resonant inWhichcase all points along its length will be very nearly at thepotential oft he adjacent space even if the strut be "actually parallel,instead of perpendicular to the field. minimize distortion of thefie'l'dand pick-up of energy from itby the strut. "This condition can ,at bestbe approximated in practice- Since the tube is tunable, the maximumimpedance condition for the strut wm not be met at all ire quencieswithin the normal tuning range. If the" strut be so dimensioned andpositioned that its effect is nil at approximately the ceuter of therange, the capacities etfectivelyiadded :toor subtracted from those ofthe acceleratorwill be proporti'onatelysmall at any operating frequency;therefore neither'the tuning range nor the potentials appearing'onjtheaccelerator will be seriously a'fl'ected. Hence it is not necessary thatthe condition of zero, e'fi'ect on frequency be met at the precisefrequency chosen for test; if the effect is small enough, it will bezero. at v [some closely. adjacent frequency and within tolerance at allfrequencies within the tuning range.

There are various combinationsof operating frequency, accelerating,potential and, hence, transit time at which the device may be operated.Two such combinations deserve especial-comments In accordance with thefirst. the accelerating potential is so adjusted as to make the transittime across the cavity substantially equal to /2 cycleof the resonantfrequency, for electrons, entering the cavity at optimum phase. Underthese circumstances, such. electrons are delivering energy to theoscillating field duringtheir entire transit, and this might appear tobe the optimum condition, of operation. -It results, however, in a'haddistribution of electronflow; the variation in electron velocity d'uetothe superposition of the oscillating field on the accelerating fieldresults in a relatively wide disparity in time of arrivalof'electrons atthe anode, even though the bias in the cathode-grid cavity be soadjusted that most of the electrons enter the anode cavity atapproximately the optimum epoch of the 'cycle,,the instant of reversalof potential within it. The result is that many electrons absorb-energyfrom the field instead of delivering energy thereto, resulting inrelatively low anode 'efiiciency in the device.

Ineifect this mode of operation results in un-bunching of electrons andan effective reduction in the degree of density modulation imparted tothe electron stream by the grid. Such 'a result is unfavorable; anopposite and favorable effect can be secured by operating the tube inthe manner next to be described.

The second mode of operation involves the adjustment of the acceleratingpotential so. that the transit time is one whole cycle, dividedsubstantially one-half between grid and accelerator, the other halfbetween accelerator and anode. Under these circumstances, optimum-phaseelectrons are accelerated by and therefore absorb energy from both theconstant held and the oscillating field during As has already beenstated, the oscillating potential between grid and accelerator ispreferably one-third to onehalf that between accelerator and anode. Theenergy of any electron. is directly proportional to the potentialthrough which it falls; therefore the energy absorbed from theoscillating field will be from one-half to onethird that delivered backto that field during the second part of the journey and the net energydelivered to the field will be from one-half to two-thirds that sodelivered in the latter portion of the trip. For maximum efficiency thisnet energy should be equal to that absorbed from the bias field, and theelectrons should arrive at the anode at relatively low velocity.

Such an optimum condition cannot be met in practice for all electrons;there must be some distribution in time of entry into the cavity, theamounts of energy absorbed from :and delivered to the field will varywith phase, while the D. C. energy remains a constant. Where the averagetransit time is a full cycle, however, the velocity modulation impartedto the originally density-modulated stream by the accelerating electroderesults in a bunching of the electron flow, so that many more of theelectrons therein make their transit during optimum phase conditions andenergy is delivered in short. pulses at the most eflicient epoch of thecycle. Such conditions of operation are somewhat analogous. totype Coperation of ordinary low frequency triodes, where high anode efiiciencyis secured by similarly pulsing .an oscillating circuit at optimumphase.

In order to achieve such operation, the spacing of the accelerator mustbe so proportioned as properly to divide the transit time between theelectrodes, taking into account the varying acceleration in thegrid-accelerator space and deceleration in the accelerator-anode space.This spacing must be reconciled with the necessary differences incapacities between the accelerator and the opposed cavity walls. Thelatter, however, are controllable separately from the spacing by varyingthe size and .position of the flange 7 6 and the conformation of thecavity walls opposed to the accelerator. In the tube of Fig. 1' theaccelerator and anode face each other edge to edge; 7

grid "and accelerator more nearlyflat to fianwhich' gives the desiredcapacity difl'erence. I It should not be inferred from what has beensaid howter. The two modes are discussed hereas representative of twospecial cases and not as the only modes of operation of which the deviceis capable.

In order to give some idea of the approximate dimensions and powercapabilities of tubes of the character here described, it may be statedthat that illustrated in Fig. 1 has an over-all length of 30" and iscapable of delivering an average power output of 20 kilowatts, deliveredin megawatt pulses.

The tube illustrated in Fig. 2 is capable of delivering power at a stillhigher rate and is shown to illustrate the application of the samegeneral principles to resnatrons of the annular cathode type, thegeneral principles of which are set forth in copending applicationSerial No. 771,852, above referred to. This particular tube is designedfor zero grid bias operation so that no chokes or insulators areprovided between the grid and the cathode. Without going into thedetails of constructions, since the latter are not pertinent to theinvention herein specifically claimed, the cathode 161, like that of thetube first described, is of the so-called Phillips type, formed ofsintered tungsten powder impregnated with a barium oxide and capable ofhigh-density electron emission. It is heated by electronic bombardmentfrom a small filament 103 of the ordinary thoriated tungsten type. Thecathode is annular in form, and, as already indicated, is electricallyconnected to the grid so as to operate without grid bias. The input tothe cathode-grid cavity is through a coaxial transmission line sleeve105' and a central conductor 107 which also carries a supply for coolingfluid. This transmission line may be fed by wave guide 109 and the wholearrangement is so tuned that the cathode 161 and the grid 111 whichfaces it are at the potential loop nearest the shorted periphery of thecathode-grid cavity.

An annular anode 113, having a deep electron receiving slot 115 formedtherein, is mounted coaxially with the cathode and grid so as to receivethe annular electron stream therefrom. A plurality of annular cups 117,118, 119 and 120 surround the anode structure, which is water cooledthrough channels 121, and connects with a tuning mechanism, indicated at123, for varying the capacity of the anode cavity.

The anode cavity may be considered as terminating in potential node atthe bottom of the cup 117. Viewed from Within the cavity a current nodeoccurs substantially at the lip of the cup one quarter-wavelengti1 fromthe bottom, and the outer conductor can therefore be opened at thispoint, by the space between the cup and the outer wall, withoutaffecting the mode of oscillation or permitting the escape of largeamounts of energy. Such energy as does escape through this space isattenuated by the successive cups 118, 119 and 120, as is described inthe above mentioned patent to Sloan and Marshall.

The accelerating electrode comprises two annuli 125, 125, mounted withinand without the annular path of the electron stream, on struts 127 and127' respectively. A potential node occurs substantially where the outerwall of the cavity starts to flare, and struts 127 connect to the anodestructure at the potential node just discussed, although, owing to thecomplex shape of the cavity, this is experimentally determined. They aresubstantially a quarter-wavelength long and their size is such that theyoffer a large impedance mismatch. Hence they transfer little energy tothe accelerating electrode, even though the tube be tuned to a frequencywhich will displace the nodes slightly from the position of attachmentof the strut. At the mid-frequency of oscillation of the device,however, the ideal conditions of nodal attachment are substantially metin this particular design.

In tubes of this type an additional problem enters with respect to thenumber and disposal of the struts. One of the primary reasons forexciting the accelerating electrode capacitively from the walls of thecavity is that this leads to its operation at substantially uniformpotential around its entire circumference. To the extent that energy issupplied to it through the struts this uniformity is upset;

this is bound to happen when the tuning of the cavity is off of theanti-resonant frequency of the struts and the latter borrow or lendcapacity to the accelerator. When it occurs circulating currents flowaround the accelerator rings with consequent waste of energy.

As long as the differences of potential around the accelerator rings arerelatively small in comparison with the potentials of the acceleratorwith respect to the cavity walls this condition may be tolerated, asresulting in merely a minor loss. If the potential differences aroundthe rings become greater the currents rise, causing greater losses.Moreover, the potential differences may become great enough to disturbthe uniformity of the electron stream, and under certain circumstancesthis can increase the original potential differences, so that the effectbecomes cumulative.

This problem does not arise in tubes of the type shown in Fig. l, as thesize of the accelerator electrode is so small in comparison to thewavelengths generated by the tube that no serious differences ofpotential can be set up around its periphery. In tubes of the type ofFig. 2 it may become serious if precautions are not taken to avoid it.

It is obvious that the worst condition from this point of view would bethat arising if the fundamental resonant frequency of an acceleratorring coincided with the operating frequency of the tube. This isunlikely to occur; with the accelerator ring spaced radiallyapproximately a half-wavelength from the tube axis their circumferentiallength will almost certainly be much greater than one waveiength even incomplex cavities. It would be possible for the rings to be soproportioned that they could resonate on harmonics of their fundamentalfrequencies if these harmonics were approximately the operatingfrequency of the tube, but fortunately the mechanical features of thedesign are more likely to bring the operating frequency between thethird and fourth harmonics of the resonant frequency of the outer ringand between the second and third harmonics of that of the inner one ofthe accelerating electrode; care in design can insure that harmonicresonance is absent.

The parasitic oscillations which may be excited in the rings aretherefore unlikely to be of resonant character. None the less theirmagnitude can be influenced to a very high degree by any factor whichvaries the circumferential impedance of the rings as viewed from thepoints of attachment of the struts. One such factor is the angularspacing of the struts; another is the characteristic impedance of theaccelerator rings to circulating currents. The two factors areinterdependent, but for the purpose of the present invention the formeris the more important. With the particular tube illustrated in Fig. 2the use of six struts for the outer accelerator ring and four for theinner reduced the circulating currents to negligably small values; theuse of eight struts for each ring resulted in large circulatingcurrents. Since any factor changing the impedance of the rings may alsochange the optimum number of struts the latter is best determined bycold test as above described, but if a number of struts dictated bymechanical consideration results in large parasitic currents of thischaracter it is almost always possible to so load the rings, and thuschange their characteristic impedance, as to reduce such currents totolerable proportions.

Although the tube of Fig. 2 is considerably shorter than that shown inPig. 1, being approximately 20 inches long as actually constructed, itis capable of much higher output; approximately 20 megawatts peak power.

In the tube first described the grid and anode are located on the axiswhere a potential loop must form in any mode of oscillation where themagnetic field is circumferential; The tube of Fig. 2 has grid and anodedisplaced radialiy from the axes, and in order that it may operatesarisfactorily, the cavity must oscillate in accordance with a morecomplex mode than that utilized in operating the tube of Fig. 1. For thepurpose of this specification,

modes of oscillation where the magnetic fieldsare all circular, andthenodes and loops 'in'the electric field therefore are also circular, willbe referred to as principal modes'of oscillation, to'dist'inguishsuchmodes from those having nodes and loops angularly'disposed' around thecircumference.

It will be understood that the invention as specifically described inconnection With the two forms of resnatron illustrated herewith aremerely examples, of many forms in which the invention can be utilized.Several of the resnatrons' shown in the prior patents and applicationslisted herein can obviously be modified toemploy the principles hereinset forth and many additional designs can be evolved to place anauxiliary electrode within an otherwise necessary cavity' and avoid annntuned intermediate cavity or space through which power may be Wasted.It is desired, therefore, to protect the invention as broadly as ispossible within the scope of the following claims.

I claim: I

1. An electron tube structure comprising a cavity resonator, anelectrode structure positioned within said resoof the oscillatingelectric field between the walls thereof when said resonator isoscillating at a principal mode, means for initiating an electron streamacross the cavity of said resonator substantially along the lines ofsaid field concentration, an auxiliary electrode positioned intermediatethe walls of said cavity in line with said electron stream and aperturedto permt the passage of electrons therethrough, and conductive means forsupporting said auxiliary electrode, said conductive means beingproportioned to have a high impedance at said principal mode ofoscillation in comparison with the capacitive impedance of sad auxiliaryelectrode to the cavity walls between which. it lies, whereby theoscillatory potential of said auxiliary electrode with respect to saidwalls is determined primarily by its capacities thereto.

2. A structure as defined in claim 1 wherein said conductive meanscomprises at least one strut connected to a wall of said cavity anddimensioned to offer a mismatch of impedance with respectto the wallwhereto it is connected' sufiicicnt to cause reflection of a majorportion of oscillating energy carried by said strut.

" 3. A structure as defined in claim 1 wherein said conduc tive meanscomprises at least one strut substantially an integral number of quarterwavelengths at said principal mode of oscillation in length andconnected to the inner wall of the cavity of said resonator'at aposition of maximum impedance.

4. A structure in accordance with claim 3 wherein the length of saidstrut is substantially two quarter wavelengths and its connection withinsaid resonator is substantially at a point of maximum concentration ofelectric field.

5. A. structure in accordance with claim 3- wherein the said length ofsaid strut is substantially one quarter wavelength and its connectionwithin said resonator is substantially at a point of minimumconcentration of electric field.

6. An electron tube structure comprising a cavity resonator, an anodestructure within said resonator substantially at a pole of maximumconcentration of the oscillating electric field between the wallsthereof when said resonator is oscillating at a principal mode, meansfor initiating an electron streamacross the cavity of said resonatorsubstantially along the lines of said field concentration, an

accelerating electrode within said. cavity and having an opening thereinpositioned to permit the passage of said electron stream therethrough,and conductive supporting means connecting said accelerating electrodeto the walls of said cavity and dimensioned to present an impedance highin comparison to the capacitive impedance between 1' 2 said acceleratingelectrode and the walls of the'cavity' at the frequency of said mode.

7. An-electron tube in accordance with claim'6 wherein saidconnectingmeans comprises at least one supp'ort- 7 ing strut connectedto saidaccelerating electrode and to the interior of said resonator, thecross-sectional area vide an impedance mismatch at the points ofconnection na'tor substantially at a pole of'rn'a'xi'mum concentrationof said strut.

8. An electron tube in accordance with claim 6wherein said acceleratingelectrode comprises a conducting annulus surrounding the path of saidstream of electrons. 9. An electron tube comprising a cavity resonator,an

anode structure and a grid structure positioned in opposite walls ofsaid cavity resonator substantially at the poles of a maximum ofelectric field therein when oscillating at a principal mode, a cathodepositioned-externally of said cavity resonator to direct a stream ofelectrons through said grid structure toward saidanode structure, anelectrode so positioned within said cavity and between said poles as toaccelerate said'stream, and connections for applying an acceleratingpotential to said electrode.

10. An electron tube in accordance with claim 9 in cluding a secondcavity resonator having in common withsaid first mentioned cavityresonator the wall including said grid structure, said grid structureand said cathode being located substantially at opposite poles ofmaximum of electric field within said second cavity resonatorwhenoscillating at the same frequency as said first mentioned cavityresonator.

11. An electron tube in accordance with claim 9 wherein said electrodecomprises an annulus surrounding the path of said electron stream.

12. An electron tube in accordance with claim 9 wherein said connectionscomprise at least one supporting strut for said electrode, said strutconnecting to the interior of said resonator and mismatched in impedancetherewith.

13. An electron 'tube in accordance with claim 9 where'- in said anodeand grid structures and said cathode are coaxial and annular in form,said resonant cavity is substantially symmetrical with respect to theaxis of said structures, and said electrode comprises coaxial annulisurrounding and surrounded by, respectively, the path of said electronstream.

14. An electron tube structure comprising a cavity resonator, an anodestructure and a gridstructure within said resonator cavity on oppositesides thereof, an'electrode mounted within said cavity between saidstructures and conductive supporting means for said electrode connectedtothe wall of said cavity resonator and mismatched therewith inimpedance whereby the alternating potential of said electrode isdetermined primarily by the relative capacities thereof with respect tosaid anode and grid structures.

15. An electron tube structure in accordance with claim 14 wherein thecapacity of said electrode with respect to said grid structure ismaterially greater than its capacity with respect to said anodestructure.

16. An electron tube structure in accordance with claim 14 wherein thecapacity of said electrode with respect to said grid structure is fromtwo to three times as great as its capacity with respect to said anodestructure.

References Cited in the file of this patent UNITED STATES PATENTSLehmann Mar. 1, 1949

